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Thread: -21 Dioptres

  1. #1
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    -21 Dioptres

    If a px had -21 1.9 Hi index what power in lower index would she require as a balance to give same minification and thickness

  2. #2
    since 1964 Homer's Avatar
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    Depends ....

    How critical is your patient? A lower index would probably have less reflective qualities and therefore look a bit different. In any case, you will want to match the base curve for the same reflective reason. If you use AR you might as well duplicate the lens for the balance.

    You could also consider a blended myo-disk.

  3. #3
    OptiBoardaholic sarahr's Avatar
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    I would not recommend this. Of the top of my head you'd be looking at approximatel 6 dioptres off the power (?). Minification of the wearers features would be uneven and the weight difference would be noticable. Ther is an equation you can use to calculate the dioptral thickness of the hi-index lens which I THINK is n (1.5)/n' (1.9) X F (21D) = approx -16.50D. I stand to be corrected though, my books aren't handy!
    :cheers:

  4. #4
    Bad address email on file John R's Avatar
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    Wave

    Assuming you mean the -21.00 is in 523 index then
    1.7 = 15.69 D & 20.00 m/m @60 ---Not possible blanks too thin..
    1.8 = 13.61 D & 15.9 m/m @60
    1.9 = 12.31 D & 13.9 m/m @60

    As Sarah says you are looking at more trouble than its worth in terms of trying to match the weight and getting the AR coats the same too..You would be better getting both lenses made to the same power in the same index.
    What you are really trying to do is reduce the costs a bit, correct..

    btw, plug plug..:D If you surf over to my site i have a online conversion chart which will show the power reduction between the various indeces & a sag chart as well..All of which can be downloaded in .pdf format to be printed out.
    Hope this is of help to you..

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